1. Field of the Invention
This invention relates to a magneto-optical disk employed in an external memory of an electronic computer or a recording apparatus for recording audio or video information. More particularly, it relates to a magneto-optical disk capable of performing repeated recording and erasure, and which makes use of an amorphous alloy of rare earth-transition metals having an easy axis of magnetization in a direction perpendicular to the film surface as the recording layer.
2. Prior Art
The basic operating principle of a magneto-optical disk is that a portion of a magnetic thin film is heated to a temperature above the Curie point or a temperature compensation point to annul the coercive force of this portion, and that the direction of magnetization of this portion is inverted and assimilated with the direction of the recording magnetic field applied from outside. Therefore, in order that the magnetic domains in which the direction of magnetization has been inverted by recording will be produced with a high density and to the desired shape, it is necessary that the shape of the portion irradiated by the recording laser light be reproduced faithfully as the shape of the inverted magnetic domain. To this end, it is first requirement that the magnetic recording layer be a homogeneous amorphous thin film free of crystal boundaries.
On the other hand, in the magneto-optical disk, the magneto-optical effects, such as the magnetic Kerr effect or the Faraday effect, are utilized in reading out the magnetic domain in which the direction of magnetization has been inverted by recording. Therefore, in producing satisfactory reproducing signals, it is necessary as second requirement that the magnetization be made in the direction normal to the direction of the electro-magnetic field of the electro-magnetic waves of the reproducing laser light, that is in the direction of incidence of the laser light, and that the magnetic recording layer be a perpendicular magnetization film having its easy axis of magnetization normal to the film surface.
Heretofore a thin film of an alloy of rare earth-transition metals has been proposed as the magneto-optical recording film satisfying these requirements. The amorphous perpendicular magnetization thin film of these rare earth-transition metal alloys can be formed easily on an inexpensive base plate or substrate by vacuum film forming methods, such as the sputtering or vacuum deposition methods. Of these alloys, TbFeCo and GdTbFe are practically useful because of their larger Kerr rotation angle.
However, the rare earth-transition metal alloys are inconvenient in that they are highly susceptible to oxidation and hence to corrosion thus affecting their magnetic properties. Our experiments have revealed that, when simply exposed to atmosphere after film formation in vacuum, the TbFeCo film is oxidized to a thickness of about 50 .ANG. from its surface, so that the magnetic properties of the oxidized portion are lost. Therefore, in using the aforementioned thin films of the rare earth-transition metal alloys as the recording magnetic layer, it becomes necessary as a third requirement that the film be sandwiched on its both sides by transparent and thermally and chemically stable protective films impermeable to oxygen.
Under such a situation, there is known a magneto-optical disk wherein, as disclosed for example in the U.S. Pat. No. 4,610,912, a protective film, a magnetic recording film, and a second protective film are provided in this order on a transparent base plate; and a reflective metal film is provided as a fourth layer. This reflective metal film is provided for increasing the apparent Kerr rotation angle in conjunction with the aforementioned two protective films.
It will be noted that, in the magneto-optical disk, since the recording is achieved thermally, various recording properties are governed not only by the magnetic and optical properties, but also by thermal response characteristics. For example, the resolution capability of the recording is governed by the thermal resolution capability of the temperature increasing profile.
On the other hand, the recording/erasure properties also depend on the temperature environment at the time of the application. It is because the temperature of the recording portion is the sum of the increase in the temperature caused by the recording laser light and the temperature at which the portion is preserved. Thus the recording laser light quantity necessary for the temperature to reach the Curie point becomes larger or smaller as the preservation temperature is higher or lower. In order for the magneto-optical disk to be utilized in, for example, a computer memory, it is necessary to take account of the increase in temperature within the disk apparatus,.and the disk is required to have satisfactory characteristics under the environment of a broader temperature range of -10.degree. C. to 60.degree. C.
In addition, the recording/erasure/reproducing laser light quantity in the actual disk apparatus is subject to fluctuations caused by the errors in design values or lens contamination. It is thought to be necessary to take account of the fluctuations in excess of the range of -20% to +10%. The intensity of the magnetic field applied from the outside is also subject to fluctuations. The magneto-optical disk is requrired to be interchangeable among the disk apparatus subject to these fluctuations.
For procuring the interchangeability under the above described situations, it is necessary that the magnetic domain in which the direction of magnetization has been inverted by recording under the combination of the highest values of magnetic field, laser output, and the temperature, be able to be erased under the combination of the lowest values of magnetic field, laser output, and temperature. That is, it is necessary that the magneto-optical disk recorded under elevated temperatures and with the aid of the apparatus of the maximum light quantity and magnetic field be able to be erased by the apparatus under the lower temperatures and with the aid of the apparatus of the minimum light quantity and magnetic field. Also, when the high sensitivity disk produced with the manufacture fluctuations is erased with the combination of the highest values of the magnetic field, laser output, and temperature, the information of the adjacent tracks should not be erased by excess erasure. In addition, the reproduced information should not be lowered in quality when the same track is reproduced continuously in this state for an extended period of time.
Therefore it is necessary, as the fourth and most crucial requirement, that the magneto-optical disk have a broad area of satisfactory recordability in which the extent of the preestimated fluctuations of conditions should be sufficiently allowable.
The numerous proposals that have been made in connection with the magneto-optical disk are directed to possible advantages from the static optical and magneto-optical aspects, but not to constraints from the aspects of dynamic thermal response. Thus these proposals do not meet all of the above cited requirements and are rather poor in practical uses.
Although the four-layer magneto-optical disk has been proposed, as disclosed in the aforementioned U.S. Pat. No. 4,610,912, it has not been investigated what film thicknesses of the four films should be set for meeting the above four requirements.